The objectives of the parent grant are to quantify the therapeutic opening (gapping) of the L4/L5 and L5/S1 zygapophysial (Z) joint space following side posture positioning and side posture spinal manipulative therapy (SMT;a.k.a., adjusting) in subjects with acute low back pain. In addition, the parent grant is assessing the relationships among pain, functional impairment, and Z joint gapping. This competitive revision uniquely combines technologies from the fields of acoustics and MRI imaging to deepen the research of the parent grant by allowing further investigation of one of the mechanisms of action of SMT, namely the relationship between cavitation (audible release) and Z joint gapping following SMT. In addition, if cavitation is found to correlate with Z joint gapping, cavitation could be used as a simple, inexpensive, and quantified standard for SMT administered in clinical trials and possibly in clinical practice. The specific aims of this study are to correlate cavitation (audible release) that occurs (1) during SMT and (2) during side posture positioning, with gapping of the L3-S1 Z joints in 40 healthy subjects. The collaborating investigators will use established methods, combined with those tested in a feasibility study, to complete the research. Gapping will be calculated from the difference of Z joint space measurements made from MRI scans taken before and after SMT (adjusting group) or before and after lumbar side posture positioning (control group). The vertebral level and side of cavitation will be determined during the adjustment or side posture positioning by using data received from one linear array and two triangular arrays of accelerometers. Data analysis will determine the relationship between cavitation and gapping by comparing Z joints that: cavitate following adjusting, do not cavitate following adjusting, cavitate following side posture positioning, and do not cavitate following side posture positioning. Further work will be conducted to assess the intensity and unique acoustic characteristics of cavitations and other Z joint sounds of healthy subjects. This research is consistent with the NCCAM Strategic Plan to deepen understanding of manual therapies by helping to determine the significance of cavitation to joint movement during spinal adjusting. If a positive relationship is found, cavitation could possibly be used to identify patients who would respond optimally to high velocity low amplitude adjusting. If a negative relationship is found then cavitation would not be used as an indicator of joint movement during spinal adjusting. Consequently, regardless of the outcome, this research will deepen the understanding of SMT and manual therapies. PUBLIC HEALTH RELEVANCE: This project will deepen the research of the parent grant by further investigating one of the mechanisms of action of spinal manipulative therapy (SMT, or spinal adjusting), that is the relationship between cavitation (audible release) and gapping (separation) of the zygapophysial joint (Z joint) spaces following SMT. Although cavitation and gapping are thought to be related, there has been no evidence to support this hypothesis. If a positive relationship is found, cavitation could be used to identify patients who would respond optimally to high velocity low amplitude SMT, whereas no or a negative relationship would provide evidence that cavitation is not a suitable indicator of joint movement during spinal adjusting.